Effect of Chloride Concentration on Gold Recovery in Pressure Oxidation Process

1. Effect of Chloride Concentration on Gold Recovery in Pressure Oxidation Process Ida Fok & Amy Ng (speaker), HATCH October 24, 2007

2. Agenda • Background • Overview of Gold Recovery Process • Pressure Oxidation Process • Technical Problem • Hypothesis & Objective of Study • Findings from OLI • Conclusions

3. Background • Ida Fok • Senior Process Engineer • Over 32 years of experience in process design, control and simulation in mining and petroleum refining • Hatch global consultant for METSIM • Amy Ng • Process Engineer • Over 3 years of experience in basic and detailed design of alumina, potash, and high-pressure nickel/cobalt and gold recovery plants • HATCH • Global engineering consulting company specializing in mining, metallurgical, energy and infrastructure industries • Autoclave Technology Group, a multi-discipline design group specializing in the design of high-pressure metals recovery plants

4. Gold Ores Refractory? No Yes Pressure Oxidation Cyanidation Carbon Adsorption Reduction Overview of Gold Recovery Process NaCN Activated Carbon

5. Cyanidation • Cyanide leaching • Alkaline ore slurry is combined with sodium cyanide • Cyanide forms a complex with gold which dissolves into solution: • 4 Au + 8 NaCN + O2 + 2 H2O = • 4 NaAu(CN)2 + 4 NaOH Gold Ores Refractory? No Cyanidation Carbon Adsorption Reduction

6. Gold Ores Refractory? No Cyanidation Carbon Adsorption Reduction Carbon Adsorption • Gold-rich solution is mixed with activated carbon pellets • ‘Loaded’ carbon is washed with hot cyanide solution to strip gold from pellets

7. Reduction Gold Ores • Gold stripped from carbon pellets is recovered by electrolysis • Gold accumulates on steel wool cathodes, which are dissolved in HCl acid • Gold is ready for smelting (extraction process) Refractory? No Cyanidation Carbon Adsorption Reduction

8. Gold Ores Refractory? No Yes Pressure Oxidation Cyanidation Carbon Adsorption Reduction Pressure Oxidation Process • Pre-treatment of “refractory” gold ores for cyanidation to be effective • Oxidation of sulfides, such as pyrite (FeS2) and marcasite (FeS2) at high T and P in autoclaves • Release of encapsulated gold grains

9. Autoclaves Commercial-sized autoclave in a gold recovery plant Autoclave in a copper demonstration plant

10. Technical Problem • Testwork data shows that gold recovery is highly dependent on chloride concentration • Study of dissolution of gold to gold chloride (AuCl4-) is required through: • Literature review • OLI Corrosion Analyzer Version 2.0.52

11. Hypothesis • Dissolution of gold (Auo oxidized to +3 state to form AuCl4- in the presence of chloride ions) in acidic pressure oxidation is dependent on chloride concentration.

12. Objective • To verify the hypothesis using Au-Cl-H2O stability diagrams generated by OLI Corrosion Analyzer Version 2.0

13. Au-Cl-H2O Stability Diagram from OLI

14. Au-Cl-H2O Stability Diagram from OLI Au Recovery (Cl = 50 ppm, Acid = 10 g/l, Ratio of Fe+2/Fe+3 = 1

15. Data from OLI

16. Findings • The higher the Cl concentration, the lower the reduction potential (E) of AuCl4-/Au • Oxidation of Auo is more favourable due to common ion effect, according to this reaction: AuCl4- + 3 e- Auo + 4 Cl- • Study extended to effect of temperature, ferrous/ferric ratio and H2SO4 concentration

17. Conclusions • Reduction potential (E) of AuCl4-/Au decreases, i.e. dissolution tendency increases, with increasing temperature and chloride concentration • OLI Corrosion Analyzer proves to be a useful tool in solving this technical problem • Findings based on OLI stability diagram are consistent with those from literature

18. Questions or Comments?